1. Field of the Invention
The present invention relates to a fixing apparatus used with an image forming apparatus such as a copying machine, a laser printer, a facsimile and the like.
2. Related Background Art
For example, among fixing apparatuses used with an image forming apparatus such as a copying machine, a laser printer, a facsimile and the like, there is a fixing apparatus of heat roller type.
Such a fixing apparatus of heat roller type includes a heat roller (fixing roller) heated by an internal heat source such as a halogen heater to be maintained to a predetermined temperature, and a pressing roller having elasticity and urged against the heat roller. A recording material is introduced into a nip (fixing nip portion) between the heat roller and the pressing roller to be pinched therebetween and to be conveyed, thereby thermally fixing a non-fixed toner image to the recording material.
Recently, there have been proposed thermal fixing apparatuses of film heating type (for example, refer to Japanese Patent Application Laid-open Nos. 63-313182, 1-263679, 2-157878, 4-44075 to 4-44083 and 4-204980 to 4-204989.
The thermal fixing apparatuses of film heating type generally includes a heating body secured to a support member, a heat-resistive film rotating while contacting with the heating body, and a pressing roller cooperating with the heating body to form a nip therebetween with the interposition of the film. By pinching and conveying a recording material bearing a toner image thereon by the nip, the toner image is thermally fixed to the recording material.
The thermal fixing apparatuses of film heating type not only is utilized as an apparatus for thermally fixing a non-fixed toner image to a surface of the recording material to form a permanently fixed image but also is widely used as an apparatus for heating the recording material bearing the toner image thereon to improve a surface feature of the recording material, an apparatus for effecting a pre-fixing process, an apparatus for effecting a sheet heating process, and other apparatuses.
In thermal fixing apparatuses of film heating type, as the heating body, a heater having low heat capacity and capable of being quickly heated, for example a so-called ceramic heater comprised of a ceramic substrate having good insulating and heat conducting ability, and a resistance heat generating layer provided on the substrate and adapted to generate eat by energization can be used. Further, since a thin film material having low heat capacity can be used as the film, a temperature of the heating body can be increased for a short time, with the result that it is not required for supplying an electric power in a stand-by condition. Accordingly, even when a recording material to be heated is immediately introduced into the fixing apparatus, the heating body can sufficiently be heated up to a predetermined temperature before the recording sheet reaches the fixing nip portion, thereby reducing a wait time (achieving quick-start ability and on demand operation), saving the electric power and suppressing increase in temperature of the interior of a main body of the image forming apparatus.
It is ideal that the non-fixed toner image born on the surface of the recording material is moderately heated and melted all over and is fixed to the recording material. However, if there is cold offset toner (not adequately melted) or hot offset toner (excessively melted), such toner will be transferred to the fixing roller and the fixing film which are contacted with the surface of the recording material.
When the temperatures of upper and lower rotary members are the same, the offset toner is transferred to one of the rotary members which has poor mold releasing ability. On the other hand, when the temperatures of the upper and lower rotary members are different from each other, the offset toner is apt to be transferred to the rotary member having lower temperature because the toner is solidified on the lower temperature rotary member more easily. Particularly in the fixing apparatus having low heat capacity such as the film heating type, since the heating body is not energized in the print waiting (stand-by) condition, the pressing roller is in a cold condition. Accordingly, when the print is started from this condition, although the heating body and the film are heated by energization, since the pressing roller is still cold. In this condition, when the film and the pressure roller are rotated, the toner pinched by the nip is transferred to the pressing roller. If such toner is accumulated, hardness and/or mold releasing ability of the rotary member are changed to affect a bad influence upon the fixing ability.